Ultrasonic welding is a bonding process used extensively for bonding plastic materials to one another. Application of concentrated, directed sound waves in the ultrasonic frequency range (i.e., about 10,000 to about 70,000 kiloHertz) to a pair of compatible (i.e., physically and chemically similar) thermoplastic materials in contact with one another leads to a fusion of the contacting surfaces of the thermoplastic materials to form a bond between the materials. The strength of the bond between the materials can be controlled by the ultrasonic energy applied to the interface between the materials.
In a typical ultrasonic welding process, a vibrating metal tool, typically referred to as a “horn”, is placed over a region where two compatible thermoplastic materials are in contact. One of the thermoplastic materials rests against a relatively high mass substrate, such as a plate or roller, typically referred to as an “anvil”, while the horn is held over the other material opposite the anvil, which acts as a reflector to locally concentrate the energy of the ultrasonic waves in the materials in the region immediately between the anvil and the horn. The horn vibrates at ultrasonic frequencies and the sound waves are transmitted to the thermoplastic materials, either through the air or by direct contact of the horn with the materials. The vibrational energy from the sound waves forces the thermoplastic materials to fuse. The distance between the horn and the thermoplastic materials, the vibrational frequency of the horn, and the thermal properties (e.g., melting point, glass transition temperature, and the like) of the thermoplastic materials can be used to vary the strength of the bond that is formed. Inclusion of raised or depressed areas on the anvil helps to concentrate the ultrasonic energy in the region of the materials over the rased portion of the anvil. This typically results in a stronger weld than that obtained with a uniform “flat” anvil. In some applications the horn is momentarily brought into contact with the materials (referred to as the “plunge method”), whereas in many other applications the horn does not contact the materials at all.
Ultrasonic welding can be applied to moving webs of two compatible thermoplastic materials using a fixed position, vibrating horn and a roller as the anvil. For example, fabrics of thermoplastic fibers and/or thermoplastic films or sheets can be bonded together to form a laminate. If the anvil is provided with locally raised areas on its surface, as described above, such as an array of bars or nubs, an array of intermittent ultrasonic spot-welds can be provided between the materials. This technique has been used to form a two-layer, quilted fabric, such as is frequently used for disposable hospital gowns and diapers. Similarly, use of a narrow roller with projecting teeth as the anvil provides a linear array of spot-welds to thermally “stitch” two thermoplastic sheets together, in a pattern similar to sewn stitches, but without the use of thread and complex sewing machine mechanisms.
Ultrasonic welding provides a bond between thermoplastic materials without clamping or pressing the materials together. Simple contact between materials is sufficient for ultrasonic bonding to take place. Because ultrasonic welding typically involves using an array of localized welds, the overall thickness of the bonded thermoplastic materials is generally maintained in the regions adjacent to welded regions. In contrast, direct thermal bonding of thermoplastic materials generally requires the materials to be clamped or firmly pressed together with significant force for bonding to occur. In the case of laminated sheet materials, the resulting thermally bonded region can be significantly thinner over a larger area than the combined thickness of the two materials compared to ultrasonic welds of the same strength and distributed over the same surface area. Thus, ultrasonically bonded materials generally can be prepared with less deformation of the materials than is obtained with thermal bonding. In addition, ultrasonic bonding generally requires less energy, over all, than thermal bonding, to afford a bonds of similar strength.
Preferably, ultrasonic welding is performed with a patterned anvil, such as an anvil having an array of raised nubs, raised bars, or a combination thereof, usually arranged in a pattern on the anvil. The resulting ultrasonic weld has a pattern of bonded portions, corresponding to regions where there was a raised structure on the anvil, along with non-bonded regions interspersed with the bonded portions. The macroscopic peel-strength of the bond between two ultrasonically welded sheet materials depends on the size and number of the welds between the materials, the strength of the individual welds, the shear-strength of the sheet materials, and the like, as is well known in the ultrasonic welding art.
Ultrasonic welds also avoid excessive melting of layers that typically occurs with conventional thermal bonding processes, particularly when a patterned anvil is used. The publication Ultrasonic Plastics Assembly published by Branson Sonic Power Co., Danbury, Conn., (1979), the disclosures of which are incorporated herein by reference, provides an overview of ultrasonic welding as applied to polymeric materials.
It is common practice to seal a container with a sheet material, such as paper, a polymeric film, aluminum foil, or a laminate of paper, polymeric film and/or aluminum foil. The use of such seals, in many cases, has been imposed on the packaging industry by FDA regulations, as a protection against product tampering. Such seals can provide evidence of product tampering, since they are typically destroyed by the process of removing the seal.
It is also common to line the inner surface of container closures with a moderately compressible material, such as a polymeric material, pulp board, or a multilayer laminated combination thereof. When a closure containing the liner material is secured to the finish of a container, such as by applying a torque force to a threaded closure that is engaged with a threaded container finish, the resulting pressure exerted by the closure onto the liner, which is interposed between the closure and the container finish, produces a substantially liquid and/or gas-tight seal. When the closure is removed from the container, the liner remains within the closure. Re-engaging the closure with the container finish reestablishes the seal. Liner materials can utilize a pulp or paper substrate or polymeric materials, such as polyolefin foams or laminated multilayer lining materials comprising a combination of pulp or a polymeric foam along with a polymeric film, metal foil, and the like.
In a typical application, closures for containers are lined with a “two-piece” laminated material having a layer of pulp mounted to a layer of aluminum foil by an intermediate frangible or absorbable adhesive layer (e.g. a layer of wax or other relatively weak adhesive). Such laminated materials also frequently contain a layer of polymer, such as a polyester film, fixed by an adhesive to the foil, and a layer of heat-sealable polymer fixed by an adhesive to the polyester film, coated thereon, or extruded thereon. The laminate is produced and shipped in roll form, which is then cut to the required shape and size, and mounted in a closure with an adhesive or by friction. One portion of the laminate acts as a closure liner, whereas another portion acts as a seal for a container. The wax or weak adhesive bond between the liner portion and the seal portion is designed so that the liner and the seal will separate during the container-sealing process or upon removal of the closure from the container.
In use, a closure lined with a two-piece sealing material is torqued onto a container, such as a bottle or jar, which has been filled with a fluid or solid product. Next, the capped container is passed through a high frequency induction heating unit. During induction heating, radio frequency energy heats the aluminum foil to a temperature in excess of about 65° C., generally about 150° C. or greater.
The resulting heat melts the wax in the layer between the pulp and aluminum foil.
The melted wax is absorbed by the pulp, causing the pulp to separate from the remainder of the material. The sealing material typically is selected to match the material of construction of the container, and is heat-welded (i.e., heat-sealed) to the finish of the container (i.e., the rim around the access opening of the container) utilizing the heat generated from the induction heating of the aluminum foil. Alternatively, the seal can be affixed over the access opening of a container by an adhesive, in which case the sealing material need not be a heat-sealable polymer, and the container is sealed without recourse to induction heating. When a consumer removes the closure from the container, the pulp layer remains in the closure as a liner, leaving the laminated combination of foil, polymer film, and sealing material over the access opening of the container as seal, to provide evidence of tampering and/or to prevent leakage and contamination of the container contents during storage and shipment. To access the contents of the container, the consumer must pierce the seal to remove it from the container.
Conventional two-piece container seals (i.e., liner and seal combinations) typically use a wax layer in contact with a wax absorbent surface or a weak adhesive cause the liner portion and the seal portion to separate during the sealing process or upon removal of the closure from a sealed container. Accordingly, there is generally a residue of adhesive (e.g., a wax) remaining on the liner portion after separation. Upon re-sealing the container with the lined closure, the contents of the container come into direct contact with the residue. The contact between the adhesive residue and the container contents can be objectionable in some applications, e.g., where the contents of the container includes a solvent capable of dissolving the adhesive, leading to contamination of the contents.
The two-piece container seals of the present invention provide a liner and seal combination having an adhesive-free interface between the liner and seal portions.